Following infection of the host with Leishmania major (Lm, the etiological agent of cutaneous leishmaniasis, CL), macrophages become infected with the parasite and Lm-specific T cells are elicited. In the murine model for CL, Lm-specific Th1 cells protect mice from infection whereas Lm-specific Th2 cells exacerbate disease. To design an effective vaccine against CL (i.e., one that will not induce Th2-type T cells), the mechanisms by which Lm-specific Th1 and Th2 cells are activated must be fully understood. Such studies would expand our knowledge regarding the pathogenesis of CL and should add to our understanding of immunoregulation in general. This application will explore the interactions that occur between Lm and antigen presenting cells (APC) as well as between infected APC and parasite-specific T cells with the goal of identifying the factors [APC functions, cytokines, parasite antigens (Ag)] that determine whether Th1 or Th2 parasite-specific T cells are stimulated. Three approaches will be taken: 1) A mouse radiation chimera system was developed in which it was observed that T cells from resistant mice are unable to mediate protection when operating in a susceptible environment while T cells from susceptible mice are capable of mediating protection when operating in a resistant environment. The simplest interpretation of these results is that APC dictate whether protective or exacerbative T cells are activated. Therefore, this radiation chimera system will be analyzed further by constructing chimeras in which the animals are chimeric at the level of the B cell or macrophages to determine whether either of these cells is responsible for selective activation of Th1 or Th2 cells. 2) To further analyze the interactions that occur between T cells and APC in CL, a primary in vitro (PIV) response specific for Lm was developed. The assay will be utilized to analyze the parameters leading to activation of Lm-specific T cells by varying the APC used to stimulate the response, the parasite Ag available to the APC and the cytokines present during the generation of the response. 3) Finally, recent experiments are revealing that infected macrophages act as more efficient APC for inducing T cell activation than uninfected macrophages. Because of these observations and because Lm is an obligate intracellular parasite of macrophages, it is important to determine the effects that infection with Lm has on macrophages APC functions. Therefore, macrophages will be infected with Lm, and the following macrophages APC functions will be examined: Ag processing, the production of costimulatory signals for T cells, and the expression of surface molecules relevant to APC function.